1. Field of the Invention
The present invention relates to an impeller for transporting dirty liquid. More specifically, the present invention relates to an impeller having transport elements disposed on a pressure side of a cover disk of the impeller.
2. Discussion of the Related Art
In centrifugal pumps which transport dirty liquids, it is absolutely essential to keep the space on the pressure side of the impeller free of admixtures of the liquid that is being transported to prevent deposits from baking on solidly and to minimize the wear on seals. To achieve this goal, the cover disk of the impeller of such a pump conventionally has a large number of vanes disposed on the back or pressure side of the impeller. These vanes are distributed symmetrically over the surface of the cover disk, and rotate at a slight distance from the housing wall. The vanes on the back of the impeller cause the circumferential component of the liquid stream on the pressure side of the wheel to match the circumferential speed of the impeller. The vanes generate a more or less strong radial pressure gradient, which is always directed concentrically with respect to the impeller. The radial pressure gradient is supposed to prevent dirt and fibers from penetrating into the rear hub region of the impeller. However, a secondary stream also results. The primary stream is directed radially outward near the cover disk and the secondary stream is directed radially inward near the housing wall. This stream transports radially outward such solids whose density is greater than the average density of the transport medium. However, solids whose density is less than the average density of the transport medium are transported radially inward. Once such solids have reached the hub region of the impeller, where there is a region of low pressure, they can no longer be seized by the primary stream of the back-vanes, and cannot be transported to the periphery of the impeller. These solids therefore remain in the hub region, where they can cause a great deal of trouble. Increased wear, caking, and baked-on adhesions can thus occur in the hub region which creates the risk that the pump rotor will be blocked and that the seal will fail.
It is an object of the present invention to provide an impeller which prevents the creation of a region where depositions of solids are possible.